FIG. 2 is a circuit diagram showing a conventional example of an integrated-circuit-type temperature sensor. The integrated-circuit-type temperature sensor 1a shown in this figure includes two npn-type transistors Q1 and Q2 having different emitter areas (here, the area ratio is 1:10), and achieves detection of the ambient temperature T on the basis of the voltage difference ΔVF (=VBE1−VBE2) between the base-emitter voltages VBE1 and VBE2 of the two transistors Q1 and Q2.
The collectors of the transistors Q1 and Q2 are connected, via constant current sources I1 and I2 respectively, to a supply voltage line so that constant currents ic1 and ic2 are fed to their collectors. The collector of the transistor Q1 is also connected to the base of an npn-type transistor Q4. The emitters of the transistors Q1 and Q2 are connected together, and their node is connected via a constant current source 13 to ground. The base of the transistor Q1 is connected to the emitter of the transistor Q4, and is also to one end of a resistor R1. The base of the transistor Q2 is connected to the other end of the resistor R1, and is also connected via a resistor R2 to ground. The collector of the transistor Q4 is connected to the supply voltage line.
The integrated-circuit-type temperature sensor 1a shown in FIG. 1 is so configured that the base of the transistor Q1 serves as an output terminal Tout, from which an output voltage Vout is obtained. Accordingly, the output voltage Vout is given by formula (1) below.
                    Vout        =                                                            R1                +                R2                            R1                        ·            Δ                    ⁢                                          ⁢          VF                                    (        1        )            
On the basis of the diode equation, the voltage difference ΔVF included in formula (1) above can be unfurled to an expression given by formula (2) below.
                                                                        Δ                ⁢                                                                  ⁢                VF                            =                            ⁢                              VBE1                -                VBE2                                                                                        =                            ⁢                                                                                          k                      ·                      T                                        q                                    ·                                      ln                    [                                          ic1                                              1                        ·                                                  (                                                      1                            +                                                          VCE1                                                              V                                ⁢                                                                                                                                  ⁢                                A                                                                                                              )                                                ·                        IS                                                              ]                                                  -                                                                            k                      ·                      T                                        q                                    ·                                      ln                    [                                          ic2                                              1                        ·                                                  (                                                      1                            +                                                          VCE2                                                              V                                ⁢                                                                                                                                  ⁢                                A                                                                                                              )                                                ·                        IS                                                              ]                                                                                                                                          =                                ⁢                                                                            k                      ·                      T                                        q                                    ·                                      ln                    ⁡                                          [                                                                        10                          ·                                                      ic1                            ic2                                                                          ⁢                                                                              (                                                          1                              +                                                              VCE2                                                                  V                                  ⁢                                                                                                                                          ⁢                                  A                                                                                                                      )                                                                                (                                                          1                              +                                                              VCE1                                                                  V                                  ⁢                                                                                                                                          ⁢                                  A                                                                                                                      )                                                                                              ]                                                                                  ⁢                                                                                                      (        2        )            
In formula (2) above, k represents the Boltzmann constant; T represents the ambient temperature (absolute temperature); q represents the electric charge of an electron; ic1 and ic2 represent the collector currents of the transistors Q1 and Q2; VCE1 and VCE2 represent the collector-emitter voltages of the transistors Q1 and Q2; VA represents the Early voltage of the transistors Q1 and Q2; and IS represents the leak current of the transistors Q1 and Q2.
As will be understood from formula (2) above, the voltage difference ΔVF between the base-emitter voltages VBE1 and VBE2 of the transistors Q1 and Q2, which have different emitter current densities, varies according to the ambient temperature T. Moreover, on the basis of the correlation expressed by formula (1) above, the output voltage Vout of the integrated-circuit-type temperature sensor 1a also varies according to the ambient temperature T.
FIG. 3 is a correlation diagram showing the dependence of the voltage difference ΔVF (or the output voltage Vout) on the ambient temperature T. In this figure, the horizontal axis represents the ambient temperature T, and the vertical axis represents the voltage difference ΔVF (or the output voltage Vout). Incidentally, in a case where the collector currents ic1 and ic2 of the transistors Q1 and Q2 are equal and their collector-emitter voltages VCE1 and VCE2 are equal, the voltage difference ΔVF varies, as expressed by formula (3) below, in an ideal manner, i.e., in proportion to the ambient temperature T (as indicated by a broken line in the figure).
                              Δ          ⁢                                          ⁢          VF                =                                            k              ·              T                        q                    ·                      ln            ⁢            10                                              (        3        )            
Indeed, the integrated-circuit-type temperature sensor 1a configured as described above permits the ambient temperature T to be detected with a certain degree of high accuracy on the basis of the output voltage Vout appearing at the output terminal Tout, and thus can be used as a temperature detecting means in various apparatuses.
However, in the integrated-circuit-type temperature sensor 1a configured as described above, the currents flowing through the collectors of the transistors Q1 and Q2 vary in different manners. Accordingly, the collector current ratio ic1/ic2 between the transistors Q1 and Q2 and the collect-emitter voltage ratio VCE1/VCE2 between them vary according to the ambient temperature T, producing errors. Thus, disadvantageously, as indicated by a solid line in FIG. 3, the voltage difference ΔVF deviates irregularly from the ideal straight line expressed by formula (3) above. For this reason, the integrated-circuit-type temperature sensor 1a configured as described above often proves to be unsuitable as a temperature sensor for use in a controlling or driving apparatus for an electronic apparatus (for example, a hard disk drive apparatus or DVD drive apparatus incorporated in a car navigation system) that needs to detect the ambient temperature with high accuracy and high linearity.